Field of the Invention
This disclosure relates to a developing unit which develops an electrostatic latent image formed on an image carrier, such as a photoconductive drum, by using a developer including a non-magnetic toner and a magnetic carrier.
Description of the Related Art
In an image forming apparatus, such as a copying machine, a printer, or a facsimile machine, which is an electro-photographic type or an electrostatic recording type, and a multi-purpose peripheral which has a plurality of functions of these apparatuses, visualization (developing) is performed by adhering the developer to an electrostatic latent image which is formed on the image carrier, such as a photoconductive drum. In the developing unit which is used in such developing, in the related art, a technology of using a two-component developer (hereinafter, referred to as a developer) which is made of a toner having non-magnetic particles and a carrier having magnetic particles, is known.
In such a developing unit, the developer is born on a front surface of a developing sleeve which has a magnet disposed on an inner side, and the developer is conveyed by rotation of the developing sleeve. The amount (thickness of a layer) of the developer is regulated by a regulating blade which is disposed in the vicinity of the developing sleeve, and the developer is conveyed to a developing area which opposes the photoconductive drum. Then, the electrostatic latent image which is formed on the photoconductive drum is developed by the toner in the developer.
In addition, in general, a magnet which is disposed on an inner side of the developing sleeve includes a drawing-up pole which draws up and bears the developer in the developing container by the developing sleeve, and a cut pole which is disposed to be adjacent to the drawing-up pole and to be in the vicinity of the regulating blade. The developer which is drawn up by the drawing-up pole is conveyed to the cut pole by the rotation of the developing sleeve, and the thickness of the layer is regulated by the regulating blade. However, at this time, a shear (compression) is applied from the drawing-up pole to the vicinity of the cut pole in the vicinity of the developing sleeve. When the developer receives the compression over a long period of time, there is a possibility that the developer deteriorates, the toner is not uniformly placed with respect to the electrostatic latent image, a uniform transfer is not performed, roughness, such as graininess, is generated on the image, and image density deteriorates. In addition, as the toner is extremely pressed against the developing sleeve, there is a possibility that an uneven density is generated by fusion of the toner with the developing sleeve.
Here, as described in Japanese Patent Application Laid-open No. H11-24407, a configuration in which a drawing-up magnetic pole is provided in the vicinity of the regulating blade and regulation of thickness of the layer of the developer on the developing sleeve is performed by this one magnetic pole, is suggested. As the developer is drawn up and the thickness of the layer is regulated by the one magnetic pole, it is possible to reduce the above-described compression of the developer, and to suppress deterioration of the developer.
However, similar to the above-described technology, when the developer is drawn up and the thickness of the layer is regulated by the one magnetic pole, a magnetic force becomes weak due to a magnet in the vicinity of the developing sleeve. For this reason, there is a possibility that the amount of the developer upstream of the regulating blade becomes unstable, for example, the amount of the developer extremely decreases. Accordingly, there is a case where the amount of the developer which is born on the developing sleeve becomes unstable (coating defect of the developer is generated), and uneven image density caused by the coating defect is generated.
Meanwhile, in a configuration in which two poles, such as the drawing-up pole and the cut pole, are provided, it is considered that deterioration of the developer is reduced by lowering the magnetic force of the drawing-up pole and the cut pole. However, when the magnetic force of the drawing-up pole and the cut pole is simply lowered, similarly to the configuration of the above-described technology, the amount of the developer upstream of the regulating blade extremely decreases, and uneven image density caused by the coating defect of the developer is likely to be generated.